Video Error Concealment is one of the most commonly identified preservation issues in the digitization of DV content from tape to file. These issues are often described as blocking, glitchiness, or described as portions where groups of pixels have been shifted out of their correct position.
This file presents a very apparent use of video error concealment. On nearly every frame video data from one frame is copied to cover an error in a subsequent frame.
As the decks reads tape-based data it utilizes parity data to ensure that it is reading the tape correctly. If the data is read improperly and cannot be recovered (in the very short amount of time that the decks reads that section), then it will note the use of concealment and strategy applied to the dv stream that is output from the deck.
Strategies employed to conceal the missing video data include (the letter represent the hexadecimal value used in the dv stream to categorize error concealment strategies):
Type "2" or "A": Replace area with the same corresponding pixels of the previous frame (most common).
Type "4" or "C": Replace area with the same corresponding pixels of the next frame.
Type "6" or "E": Unspecified concealment
Type "F": This typically manifests as groups of grey blocks appearing where the pixels should be.
Often video error concealment strategies work so well that their use is not noticed in the resulting file, especially true in static frame shots that contain a high degree of repeated visual data from one frame to another.
When DV Analyzer is run of a DV file it will report on the occurence and extant of video error concealment so that the user can decide whether to make a second capture attempt. Commonly misaligned or unmaintained playback decks will cause a greater degree of video error concealment, so potentially trying to capture again in another deck may improve the results.
What would a DV video look like without the application of video error concealment? In order to demonstrate the effect, we took this sample file and replaced all of the compressed marco blocks (groups of pixels) that noted video error concealment with white pixels. The results demonstrates a substantially different experience from the version with concealment.
This version presents the version with error concealment and the suppression of error concealment side by side.
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